Many types of modern hydraulic machines, including forestry machines, excavators, earth moving machinery, and material handlers feature a variety of hydraulic functions. Each function may be controlled separately, but all may receive their energy from a single prime mover, such as a diesel or gasoline engine. Because it is very unlikely that all these functions will all demand full power simultaneously, it is common for the machine designer to select a prime mover of less power than the total possible hydraulic power consumption.
The designer is faced with a dilemma in selecting the relationship between engine power and installed hydraulic power consumption. If the engine is large enough to cover even very unlikely combinations of multiple hydraulic functions, the machine will be excessively expensive to build and to operate. If the engine is too small, some combinations of multiple functions will exceed the available power. This will cause the engine to slow, or even stall. This leads to a decrease in production, increased wear on the engine, and operator frustration. This dilemma is made worse by the fact that individual machines will operate in differing conditions, with differing loads, and with operators of differing skills and expectations.
Decreasing hydraulic load when engine capacity is exceeded has been tried in various forms. Most systems decrease pilot flow, destroke a variable output pump or otherwise reduce the hydraulic load when pilot pressure drops due to a decrease in engine speed and thus a decrease in the speed of the pilot pump. Other systems decrease hydraulic power demand of a main pump when secondary controls are activated, which is intended to prevent the total hydraulic load from becoming large enough to stall the engine.
Typical systems decrease, without completely eliminating, the hydraulic power demanded by the pilot valves. If the manual input is held constant or increased when engine speed drops, the engine may continue to operate at a lower speed. This results in decreased fuel efficiency, increased engine wear, continued decreased hydraulic power, and may eventually lead to engine stall.